Current is usually measured either directly with the use of a moving coil type current meter or indirectly by measuring the voltage drop across a known resistance. Each of these methods has disadvantages.
U.S. Pat. No. 3,007,106; entitled "Current Meter and Probe Therefor" and issued Oct. 31, 1961; discloses current measuring arrangement, one which uses a bridge circuit and which operates by sensing the magnetic field produced by the current being measured. The arrangement includes a core of magnetic material which can be opened to receive a current carrying conductor and which is adapted to be coupled to the magnetic fields surrounding the conductor. Windings, placed around the core, are excited at a fundamental frequency as provided by an ac source. Responsive thereto, an output voltage of twice or higher even harmonics of the exciting frequency is produced at an output of a bridge circuit. The output voltage is proportional to the magnetic flux produced by the current being measured. Hence, the output voltage may be extended through an amplifier to a phase sensitive detector whose output, in turn, feeds a current indicating galvonometer type device. The overall arrangement is stabilized by the use of negative feedback.
Unfortunately, known arrangements tend to be relatively expensive.